That's a tough situation. If I can't excuse myself as mellors suggested, I try to at least get a drink of water or something. Otherwise, try to take a few deep breaths. I haven't tried this myself, but I think those types of situations might be helped if you have a standard response when someone says something shocking or offensive. Maybe something like, "Well that's certainly an interesting statement!" or "Wow". ????

Not that I'd remember it, but it might help. It would at least buy you time.

gg